This invention relates to a non-contact information storage medium and a data transmission method for the medium, and more particularly to a medium of this type, such as a non-battery type radio card or a radio tag, and a data transmission method for the medium, used to perform communication with a data processing unit such as a card reader/writer (herein-after simply called a "reader/writer").
In the field of information processing systems, an information processing system has recently been developed, which includes a non-contact information storage medium (such as a non-battery type radio card or a radio tag) used as, for example, an entrance/leaving management system, and an information processing unit (such as a reader/writer) for performing non-contact radio communication with the non-contact information storage medium.
The non-contact information storage medium, i.e. the non-battery type radio card or the radio tag, receives a radio wave generated through the transmission antenna of the information processing unit, i.e. the reader/writer, and simultaneously subjects a signal indicative of data stored therein to slight amplitude modulation, thereby transmitting the modulated signal to the reader/writer in the form of a transmission wave.
The reader/writer receives the transmission wave from the non-battery type radio card or the radio tag, using a signal reception antenna.
More specifically, the non-contact information storage medium generally obtains power for driving its internal circuit by rectifying and smoothing a radio wave (i.e. an electromagnetic wave) from the reader/writer, using an antenna incorporated in the medium, and simultaneously transmits data stored therein, using the same antenna. To this end, the medium employs a data transmission method, such as a method for increasing a load current obtained through the antenna, or a method for changing its antenna-resonating electrostatic capacitance.
However, in the case of using, as the data transmission method, the method for increasing the load current obtained through the antenna, a great amount of load current will be lost. On the other hand, in the case of using the method for changing the antenna-resonating electrostatic capacitance, the resonance frequency of the antenna will inevitably vary. As a result, the amount of power which can be used within the non-contact information storage medium may well decrease, or the distance over which data can be transmitted may decrease. Thus, neither method is suitable for efficient data transmission between the non-contact information storage medium and the reader/writer.